Environmental Health - Toxic Substances
Daily Variation of Metals in Streams Can Confound Scientific Studies
USGS scientists have found that concentrations of some metals dissolved in stream waters fluctuate significantly on a daily cycle, and that this phenomenon occurs consistently across the northern Rocky Mountains. This suggests that assessing the levels of trace metal contamination in streams can be more complex than previously thought.
The two important characteristics of these diurnal variations are:
Diurnal variations in metal concentrations are attributed to a number of factors: sorption of metals to bed material, dissolution of metal-bearing minerals, uptake by aquatic plants, source variations, geochemical changes in the streambed, and streamflow variations. Diurnal variations in concentration for most metals have been observed in streams that have neutral to slightly alkaline pH, which is typical of most streams in the Nation.
Diurnal variations have been observed in acidic streams for some metals, such as iron. Water-quality data collected to establish baseline environmental metal concentrations, to identify important metal sources, to characterize metal contamination, and to evaluate potential effects on aquatic ecology can be confounded unless diurnal variations are considered in the study designs, and in interpreting historical data.
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